Chinese does not like trying to pronounce consecutive low (3rd) tones. As a result, one of the 3rd tones becomes a rising (2nd) tone. Language and Culture lesson 1.7 does a good job of explaining this. If you have three consecutive third tones, the middle becomes a rising tone (wo hen hao). Likewise, if you have four consecutive low tones, every other (2nd and 4th) becomes rising (wo ye hen hao). There are a few other rules involving yi and bu but I don't remember them off the top of my head.
One other rule - if you have consecutive 4th tones (falling), the second falling tone starts a slightly lower pitch than the first.